A typical blasthole drilling system includes a drill carrier or "trackdrill", an impacting device or "drifter", a drill string comprising a drill steel and couplings, and a bit. The blasthole drilling operation forms holes in a rock formation which are subsequently filled with an explosive material and detonated to fracture the rock into small pieces which can subsequently be removed.
The holes are usually drilled in a specific grid pattern. Certain circumstances, however, may cause the bit to become lodged or stuck in the hole. For instance, shifting of loose material in the hole, failure to adequately flush drilled material out of the hole, or debris falling into the hole often result in the bit becoming stuck in the hole. In these cases, the bit cannot be easily extracted. Consequently, time is lost in attempting to remove the lodged bit from the hole. In some cases, the bit and steel become lodged to the extent that removal is impractical or impossible. In these cases, the bit and steel are often left in the hole and a new hole drilled adjacent to the original hole, thereby resulting in the loss of both time and equipment.
Reverse percussion devices, which create a percussive force in a direction opposite to the percussive force generated during normal drilling, are known in the prior art. When a reverse percussion device is added to a drifter, recovery of a lodged bit and steel is facilitated by superimposing an upward repetitive impacting force on the steady upward force exerted by the feed system.
In particular, conventional reverse percussion devices operate on the principle that the reverse percussion piston, when idle, rests in a downward position. The piston is held in position by seal friction against the influence of any residual pressure or system back pressure in the reverse percussion chamber. When the operator perceives a need for reverse percussion activation, a manual control valve connects supply pressure to the reverse percussion chamber. The reverse percussion piston forces a shank adapter into the normal drilling impact position, holding the shank adapter in position with supply pressure. The drifter piston then strikes the shank adapter normally, causing the reverse percussion piston to move downward slightly in response to the impact and then return quickly to its upward position. A slight impact is created against the shank adapter collar by this action and the impact assists in retracting the stuck drill string.
When the need for the reverse percussion no longer exists, the manual control valve vents the reverse percussion chamber to a tank and the reverse percussion piston is allowed to return to its downward position, pushed by the shank adapter collar while retracting the drill string from the hole.
More particularly, known devices function by hydraulically forcing a shank adapter upward into its normal drilling position, during retraction of the bit from the hole, and causing the drifter piston to cycle normally. The shank adapter is repeatedly struck and forced downward by the drifter piston and then abruptly returned into position by the constant hydraulic force against the reverse percussion piston. This motion tends to loosen the stuck bit. However, wear on the drill string components is accelerated because the drill string connecting threads are alternately tightened and loosened with each impact cycle. In addition, these devices are subject to abuse when left operating even when the bit is not being struck. Under this condition, all energy generated by the reverse percussion operation must be dissipated in the drill string, which further aggravates the wear problem on component parts.
Such prior art reverse percussion devices, however, are somewhat destructive to drilling equipment. This is due in part to the fact that the full drifter piston energy is delivered to the drill string, but very little of the energy is actually used. Another disadvantage associated with the prior art is that such devices are subject to operator abuse since manual control allows the reverse percussion device to be activated when it is not actually necessary, thereby accelerating damage to equipment and system components.
Additionally, conventional reverse percussion devices are often sensitive to system backpressure. This is attributable to the dependence upon seal friction alone to prevent activation of the reverse percussion device under the influence of backpressure. Because the accumulator must operate over a very large pressure range from system backpressure to large pressure spikes generated by the drifter piston impact, accumulator life tends to be short. Yet another disadvantage associated with the prior art is the requirement for additional control valve components in the hydraulic system.
It would therefore be desirable to provide a reverse percussion device which overcomes the problems associated with the prior art. In particular, it would be desirable to provide a reverse percussion device having a cycling piston which eliminates wear and tear of equipment, which eliminates operator abuse and enhances the efficiency obtained from reverse percussion devices of the prior art.